The present invention is generally directed to a control system and, more specifically, to a noise reduction system for reducing noise associated with an input signal provided to the control system.
Control systems have been implemented in a wide variety of industrial and automotive applications. For example, control systems have been implemented within motor vehicles to control various subsystems of a motor vehicle. In many of these subsystems (e.g., a powertrain control module), feedback loops are often implemented to provide accurate control of the subsystem. In order to prevent noise from affecting a particular control system, low pass (LP) filters (i.e., integrators) are often utilized to reduce noise present on a sensor input signal. However, a low pass filter generally adds a phase lag to the input signal, which may reduce the responsiveness of the control system.
In automotive type systems, many of the noise sources are electrical in nature and may be caused by load switching, ignition systems and exhaust gas recirculation (EGR) coils, to name a few noise sources. Traditionally, automotive noise has been reduced by: shielding a given sensor, utilizing twisted wire pairs and/or shielded wires, implementing differential amplifiers to cancel common mode noise and/or providing a low pass (LP) input filter to filter the noise.
Two metrics that are generally utilized to provide an indication of the stability of a control system are gain margin (GM) and phase margin (PM). The difference between +/xe2x88x92180 degrees and the phase of the control system, at unity gain (i.e., 0 dB), is the phase margin of the control system. The gain margin of a control system is the additional gain that is required to provide unity gain at a phase crossover frequency. For stability reasons, it is desirable to design a control system with a relatively high PM or GM within its designed operating frequency range. However, at some frequencies, eventually all control systems tend to become unstable. A measure of the usable frequency range of a given control system is indicated by its bandwidth (BW). A control system that is subject to noise that is not eliminated in some fashion may produce a false reaction if the control system treats the noise as a system disturbance.
Therefore, it is generally desirable to eliminate noise on an input to the control system such that the noise does not detrimentally affect the operation of the control system. However, common techniques such as adding a LP filter between a sensor and an input of the control system may, as previously discussed, introduce an increased phase lag, which, in turn, can reduce the responsiveness of the control system and may cause the control system to become unstable.
For example, a noise reduction system that samples at a rate of one millisecond and averages twenty samples provides an input signal to the control system that is delayed by twenty milliseconds. Delaying the input signal is generally undesirable as a slow response time typically decreases the bandwidth of a given control system. In systems that require a faster response, one solution has been to implement a processor that can sample at a higher rate. For example, a processor that can take a sample every fifty microseconds can complete twenty samples in one millisecond. However, this requires faster, more expensive hardware, which tends to increase power consumption and heat dissipation, which may also affect system reliability. Further, such systems may not adequately function if the noise duration is relatively long. For example, if the noise spike in the above example is two milliseconds long, sampling at a faster rate merely averages the noise.
What is needed is a noise reduction system that reduces noise associated with an input signal that is provided to a control system that also substantially minimizes the adverse affect on the responsiveness and stability of the control system. It is also desirable to provide such a noise reduction system for use in a motor vehicle.
The present invention is directed to a noise reduction system for reducing noise associated with an input signal provided to a control system that also substantially minimizes the adverse affect on the responsiveness and stability of the control system. The noise reduction system includes a processor, a memory subsystem and processor executable code. The processor executable code causes the processor to perform a number of steps. Initially, the processor determines a level of an input signal at an input of the noise reduction system. Next, the processor determines a level of an output signal at an output of the noise reduction system. The processor then determines a magnitude of a difference signal that provides the difference between the input signal level and the output signal level. When the magnitude of the difference signal is less than a predetermined noise limit, the input signal is provided to an input of a control system. When the magnitude of the difference signal is greater than or equal to the predetermined noise limit, the output signal is provided to the input of the control system.
These and other features, advantages and objects of the present invention will be further understood and appreciated by those skilled in the art by reference to the following specification, claims and appended drawings.